We have identified cells in human fetal brain cultures that maintain the morphology of neural stem or progenitor cells. These cells cluster in aggregates described as neurospheres. In order to identify potential stem or progenitor cells, we have produced polyclonal antibodies to peptides from the human nestin gene. Nestin is an intermediate filament which is highly expressed in progenitor cells in the nervous system and other tissues. Immunocytochemical detection of nestin in human fetal brain cultures correlates with identification of mRNA for nestin using rt-PCR in sibling cultures. These cells also do not react with other phenotypic markers used for more mature, differentiated cells in the brain. Some of the cells in the neurospheres migrate from the clustered arrangement and appear to alter morphology depending upon the growth conditions of the cultures. Cells cultured on standard minimum essential medium supplemented with fetal bovine serum show broad cytoplasm with enlarged nuclei. These cells react strongly with antibody to glial fibrillary acidic protein, GFAP, identifying the cells as astrocytes. These cells also are susceptible to infection by the human polyomavirus, JCV, which infects cells in a glial lineage not a neuronal lineage because of cell specific gene recognition by NF-1/D transcription factor. The use of this virus to mark cells during differentiation may provide a new tool for experiments asking questions of pathways for cell differentiation in the CNS.